I knew somebody was going to say something like this. There is nothing silly or cartoony about the idea of cetacean sentience. Scientists and SF authors have recognized it for generations, to the point that many scientists now argue that dolphins should be legally recognized as people and granted the same rights as humans.
(PhysOrg.com)—Scientists studying dolphin behavior have suggested they could be the most intelligent creatures on Earth after humans, saying the size of their brains in relation to body size is larger than that of our closest relatives, the chimpanzees, and their behaviors suggest complex...
phys.org
Basically, the above article says that by every neurological and behavioral standard of intelligence we know, dolphins rate nearly equal to humans. Indeed, the article leaves out some things, like the fact that they can understand spoken English (while we still can't understand their language), the fact that they give themselves names, and the fact that in some cognitive areas, their brains seem to be even more developed than ours. According to the paper
A Comparison of Primate and Dolphin Intelligence as a Metaphor for the Validity of Comparative Studies of Intelligence, “In certain areas of the brain concerned with ’emotional control, objectivity, reality orientation, humor, logically consistent abstract thought and higher creativity’ dolphins have [a] higher ratio of neural density” than humans. So even if dolphins aren’t quite as smart as we are (and that’s far from certain), they’re probably a lot saner.
SeaQuest, after all, was a show that, in its first season before it was dumbed down, strove to be a plausible hard science fiction series whose stories were extrapolated from real science (discounting the occasional one about psychics, ghosts, or aliens -- nobody's perfect). They included Darwin
because science says that dolphins are probably comparable to humans in intelligence. Rick Sternbach (who's painted a number of SF novel covers featuring tool-using dolphins) and Michael Okuda undoubtedly established Cetacean Ops in the
TNG Tech Manual for the same reason.